Tatyana N. Andryushchenko

A Mechanistic Model for Copper Electropolishing in Phosphoric Acid

Copper electropolishing in phosphoric acid has been characterized using electroanalytical methods, primarily potential transient techniques. An uncommon voltage response, consisting of two distinct steps, was noted when the current was stepped to the limiting current. A slow (∼ 100-300 s, depending on agitation) and relatively small (∼50 mV) initial potential increase was followed by a fast (∼5 s) and large (∼1.5 V) potential rise. The latter always reached the oxygen evolution potential (∼ 1.6 V), irrespective of the process conditions. While the first, slow potential transient can be correlated in terms of a diffusion process, the second, rapid potential rise suggests the buildup of a highly resistive component, most likely a surface film. The nearly instantaneous potential relaxation upon current interruption further supports the resistive film model rather than a transport-related process. A two-stage mechanism is proposed and analytically modeled. Accordingly, the cupric ion concentration at the anode increases during the initial stage of the dissolution process due to transport limitations, until a saturation level is reached and a resistive surface film forms. During the second stage, the continuing imbalance between the rate of cupric ion formation and transport into the bulk leads to increasing film thickness and, consequently, to a rapid buildup in resistance. The model quantitatively correlates the experimentally measured transients and is consistent with all other observations relating to the copper electropolishing process.

Long wavelength roughness optimization during thin Cu film electropolish

Recent studies of Cu electropolish indicate planarization is not possible due to a large diffusion layer thickness relative to a small post-electroplate step height and small Cu overburden available for electropolish. Assuming an integration scheme that includes a CMP step followed by electropolish, the subsequent challenge of maintaining surface roughness over 300 μm range while electropolishing thin Cu films is addressed by optimizing applied current. For a typical electropolish solution of 6.4 M H 3 PO 4 , 5.4 M glycerin and 17.5 M H 2 O, an rms value of 38 A, comparable to the incoming post-CMP wafers, is demonstrated for 2500 A Cu removal.